Polycarbonate resins are used for substrates for optical information-recording media on which information data are written, read or rewritten by the use of laser rays, including, for example, audio disks, laser disks, optical memory disks, magnet optical disks, etc, because the polycarbonate resins have good moldability, good transparency, good heat resistance and good mechanical properties. In particular, the substrates for DVDs (digital video disks or digital versatile disks) shall be thinner than those for ordinary CDs. Therefore, polycarbonates for substrates for DVDs are required to have good fluidity so that they could satisfy the requirements of reduced birefringence, good transcriptional ability and good cracking resistance in molding. To increase their fluidity, polycarbonates may be so controlled as to have a lowered molecular weight. However, the problem with polycarbonates having a lowered molecular weight is that their impact resistance is poor. Therefore, polycarbonates for DVD substrates are required to have well-balanced and improved fluidity and impact resistance.
For producing polycarbonates, known are a method of directly reacting a dihydroxy compound such as a diphenol or the like with phosgene (interfacial polycondensation), and a method of transesterifying a dicarbonate with a dihydroxy compound such as a diphenol or the like in a melt phase (melt polymerization). We, the present inventors have proposed DVD substrates produced through interfacial polycondensation (see Japanese Patent Laid-Open No. 276037/1990).
However, the DVD substrates proposed are problematic in that the interfacial polycondensationmethod for them requires toxic phosgene as one reactant and is therefore unfavorable. In addition, although their properties are good in some degree, they are not yet on the satisfactory level.
One object of the present invention is to provide a process for producing polycarbonates having well-balanced fluidity and impact resistance, without requiring toxic phosgene as the reactant. Another object of the invention is to provide optical-disk substrates with reduced birefringence and improved transcriptional ability, by molding the polycarbonates produced in the process.